1. Field of the Invention
The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel, which said discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner, electrodes being arranged in the discharge space for maintaining a discharge in said discharge space, and an electrode shield surrounding at least one of the electrodes.
2. Discussion of the Prior Art
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be coated with a luminescent layer containing a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. Low-pressure mercury-vapor discharge lamps comprise a generally tubular discharge vessel which is circular in section, and which includes elongated as well as compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of comparatively short, straight parts having a comparatively small diameter, which straight parts or connected to one another by means of bridge parts or by means of, for example, arch-shaped parts. Compact fluorescent lamps are generally provided with a lamp cap (with integrated electronics).
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph is known from DE-A 1 060 991. In said known lamp, the electrode shield surrounding the electrode is made from thin sheet titanium. By using an electrode shield, which is also referred to as anode shield or cathode shield, blackening at an inner surface of the discharge vessel is counteracted. In this respect, titanium serves as the getter for chemically binding oxygen, nitrogen and/or carbon.
A drawback of the use of such an electrode shield is that the titanium in the electrode shield may amalgamate with the mercury present in the lamp and, thus, absorb mercury. As a result, the known lamp requires a relatively high dose of mercury to obtain a sufficiently long service life. Injudicious processing of the known lamp after its service life has ended adversely affects the environment.
It is an object of the invention to provide a low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph, which consumes comparatively little mercury.
In accordance with the invention, this object is achieved in that the electrode shield carries an electric current during operation, and the temperature of the electrode shield during nominal operation is above 250xc2x0 C.
In the description and the claims of the current invention, the designation xe2x80x9cnominal operationxe2x80x9d is used to indicate operating conditions in which the mercury vapor pressure is such that the radiant efficacy of the lamp is at least 80% of that during optimum operation, i.e. operating conditions in which the mercury vapor pressure is optimal.
For the proper operation of low-pressure mercury-vapor discharge lamps, the electrodes of such discharge lamps include an (emitter) material having a low so-called work function (reduction of the work function voltage) for supplying electrons to the discharge (cathode function). Known materials having a low work function are, for example, barium (Ba), strontium (Sr) and calcium (Ca). It has been observed that, during operation of low-pressure mercury-vapor discharge lamps, material (barium and strontium) of the electrode(s) is subject to evaporation. It has been found that, in general, the emitter material is deposited on the inner surface of the discharge vessel. It has further been found that Ba (and Sr) which is deposited elsewhere in the discharge vessel no longer participates in the electron-emission process. The deposited (emitter) material further forms mercury-containing compounds, for example amalgams, on the inner surface, as a result of which the quantity of mercury available for the discharge decreases (gradually), which may adversely affect the service life of the lamp. In order to compensate for such a loss of mercury during the service life of the lamp, a relatively high dose of mercury in the lamp is necessary, which is undesirable from the point of view of environmental protection.
The provision of an electrode shield, which surrounds the electrode(s) and, during operation, carries an electric current and, during nominal operation, is at a temperature above 250xc2x0 C., causes the reactivity of materials in the electrode shield relative to the mercury present in the discharge vessel, leading to the formation of amalgams (Hgxe2x80x94Ba, Hgxe2x80x94Sr), to be reduced.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in accordance with the invention in that the temperature of the electrode shield during nominal operation exceeds 450xc2x0 C.
At temperatures above 450xc2x0 C., the mercury abstracted from the discharge by amalgamation is released again. Particularly HgO and mercury-containing Ba and Sr compounds dissociate at a temperature of 450xc2x0 C. or higher. By using an electrode shield which is heated to a temperature of 450xc2x0 C. or higher, mercury is released from the compounds of mercury and oxides of emitter material. A particularly suitable temperature of the electrode shield is approximately 500xc2x0 C., at which temperature the dissociation of said compounds takes place relatively rapidly.
The known lamp comprises an electrode shield of thin sheet titanium, which material relatively readily amalgamates with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the electrode shield, which surrounds the electrode(s), reacts with mercury and/or bonds with mercury.
In operation, the electrode shield is customarily heated by the heat radiated by the electrode. It has been recognized by the inventors that it is advisable to heat up the electrode shield to the desired temperature. To achieve this, a preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in that the electrode shield electrically contacts the electrode via a current conductor.
In general, the discharge vessel of the low-pressure mercury-vapor discharge lamp is provided with a first and a second current-supply conductor, which issue from the discharge vessel (10) to the exterior. Heating up of the electrode shield preferably takes place by the first current-supply conductor electrically contacting the electrode shield and the second current-supply conductor electrically contacting the electrode. By virtue thereof, the electric current that flows to the electrode via the current-supply conductors also flows through the electrode shield.
In order to obtain an electrode shield which can be heated to such high temperatures during nominal operation of the discharge lamp and, during operation, is capable of maintaining said high temperatures throughout the service life of the discharge lamp, the electrode shield is preferably manufactured from a metal or a metal alloy which can withstand temperatures of 450xc2x0 C. or higher. An xe2x80x9celectrode shield which can withstand high temperaturesxe2x80x9d is to be taken to mean in the description of the current invention, that, during the service life of the discharge lamp and at said temperatures, the material from which the electrode shield is manufactured does not show signs of degassing and/or evaporation, which adversely affect the operation of the discharge lamp, and that no appreciable changes in shape occur in the electrode shield at such high temperatures.
In order to reach the desired temperature of the electrode shield during nominal operation of the discharge lamp, the current flowing through (a current-carrying portion of) the electrode shield, during operation, causes energy in the form of heat to be dissipated in the electrode shield. In a preferred embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention, the conductivity of the current-carrying portion of the electrode shield is such that, during nominal operation, the power dissipation lies in a range from 0.1 to 10 Watts. The electric resistance of the current-carrying portion of the electrode shield is determined by the resistivity of the material used for the current-carrying portion of the electrode shield and the dimensions (length and width, and the ratio of length to width) of the current-carrying portion. It is desirable that the current passage through the electrode determines the current intensity available for the electrode shield. In low-pressure mercury-vapor discharge lamps, the current intensity through the electrode, also referred to as the lamp current, generally lies in the range from approximately 200 mA to approximately 1 A. The power dissipation preferably lies in the range from 0.5 to 5 watts.
In a particularly favorable low-pressure mercury-vapor discharge lamp, the electrode shield is made from a ceramic material or carbon. Ceramic materials and carbon are resistant to high temperatures. Such materials have a high corrosion resistance, a relatively low coefficient of thermal conduction and a relatively poor thermal emissivity in comparison with the known materials. By virtue thereof it becomes possible to manufacture a stainless steel electrode shield, which can relatively readily reach temperatures above 450xc2x0 C. by exposure to heat originating from the electrode.
A ceramic material which is particularly suitable for the manufacture of the electrode shield is aluminum oxide. A particularly suitable electrode shield is manufactured from so-called densely sintered Al2O3, also referred to as DGA. To obtain the desired conductivity, use can also suitably be made of doped ceramic materials, such as so-called cermets (DGA doped with molybdenum and/or tungsten).
In an alternative embodiment, the current-carrying portion of the electrode shield comprises an electroconductive coating which is applied to the electrode shield. For such a conductive coating use can be made of metals, for example gold, silver or aluminum, or of carbon. For the coating use can also suitably be made of ITO (indium tin oxide), ATO (antimony tin oxide), disilicides and carbides. The thickness of the conductive coating and the resistivity of the material determine the conductivity of the material.
In a particularly favorable embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention, the electrode shield is provided, at a side facing away from the electrode, with a low-emissivity coating for reducing the radiation losses of the electrode shield. By applying such a layer to an outer surface of the electrode shield, the desired relatively high temperatures of the electrode shield can be reached more readily. The low-emissivity coating preferably comprises chromium or a noble metal, for example gold. Other materials which can suitably be used for a low-emissivity coating on the outer surface of the electrode shield are titanium nitride, chromium carbide, aluminum nitride and silicon carbide.
In an alternative embodiment of the low-pressure mercury-vapor discharge lamp, the electrode shield is polished on a side facing the discharge. Also a polishing treatment of the outer surface of the electrode shield causes the heat radiation by the electrode shield to be reduced.
In a further alternative embodiment of the low-pressure mercury-vapor discharge lamp, the electrode shield is provided, at a side facing the electrode, with an absorbing coating for absorbing radiation. By applying a layer having a relatively high emissivity in the infrared radiation range, the heat-absorbing capacity of the electrode shield is increased. By virtue thereof, the desired relatively high temperatures of the electrode shield can be reached more readily. The absorbing coating preferably comprises carbon.
Electrodes in low-pressure mercury-vapor discharge lamp are generally elongated and cylindrically symmetric, for example a coil with windings about a longitudinal axis. A tubular electrode shield harmonizes very well with such a shape of the electrode. Preferably, an axis of symmetry of the electrode shield extends substantially parallel to, or substantially coincides with, the longitudinal axis of the electrode. In the latter case, the average distance from an inside of the electrode shield to an external dimension of the electrode is at least substantially constant.
Preferably, the electrode shield is further provided with a slit. A slit in the electrode shield in the direction of the discharge increases the feasibility and/or the mounting of the shield. The slit preferably extends parallel to the axis of symmetry of the electrode shield (so-called lateral slit in the electrode shield). Preferably, the slit is directed towards the discharge space and brings about a relatively short discharge path between the electrodes of the low-pressure mercury-vapor discharge lamp. This is favorable for a high efficiency of the lamp. In the known lamp, the aperture or slit in the electrode shield faces away from the discharge space.
The electrode shield is generally held in the desired positioned around the electrode by means of a support wire, which support wire can be mounted in the discharge vessel in various ways. In an alternative embodiment of the low-pressure mercury-vapor discharge lamp, a support wire carries the electrode shield.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.